Carcinoma of the mammary gland in a mare

A 20-year-old Quarter Horse mare was presented with a firm, nonpainful swelling near the axial margin of the left mammary gland. Ultrasound examination of the mass revealed a 35mm poorly encapsulated, homogeneous mass within the parenchyma of the left mammary gland. Using histopathology and immunohistochemistry, the mass was diagnosed as a mammary carcinoma and showed positivity for cytokeratin 18 (CK18), vimentin and -smooth muscle actin. Additionally, the mRNA expression level of the oncogene cMyc did not show a significant upregulation, whereas p53, a well-known tumour suppressor gene in breast cancer, was significantly reduced in comparison with healthy equine mammary gland tissue. To the authors' knowledge, this is the first report of a significant downregulation of p53 expression in a mammary carcinoma of a mare.